The operation of gas turbine engines involves the passage of high volumes of air through the compressor portion of the engine. The high volumes of air inevitably candy a certain amount of hard particles, including abrasive materials such as sand, dirt, metal scraps, etc., which cause serious problems of erosion to the engine components.
The need to provide greater power and greater thrust-to-weight ratios in the advanced engines has led to the development of lightweight non-metallic components in certain portions of the engines. Fiber reinforced epoxy matrix composites are a typical example of materials which have found extensive application in the aerospace industry, particularly for use as airfoil materials in the fan bypass and early compressor stages, where these composites can withstand the temperatures incurred.
Experience with these non-metallic composites has shown that they are prone to surface erosion due to the abrasive particles impinging on the surfaces of the components. This erosion seriously decreases the useful lifetime of the components.
It has been customary to apply an erosion resistant coating to the surface of the components to prolong their lifetimes. A typical coating is achieved by spraying a layer of polyurethane onto the surface of the component, followed by curing at elevated temperature to bond the coat and develop the optimum erosion resistant properties. Typically several coats, up to as many as 20, have been applied to achieve a sufficient coating thickness for adequate erosion resistance. Total coating thickness is usually on the order of 0,005-0.020". Careful surface preparation of the part is required to promote coating adhesion.
The sprayed polyurethane coating has been found to be insufficient to provide adequate protection on the leading edges of the airfoils, where the erosive effects are the greatest. In these areas it is customary to install a metal sheath, either solid or mesh, to provide the necessary protection. However it is not desirable to coat the entire surface of the airfoils with metal because of the added weight and the difficulties of fabrication of the airfoils with an all encompassing metal sheath.
Murphy, et. al., in U.S. Pat. No. 4,594,761, disclose the use of a polyurethane sheath to provide erosion resistance on hollow airfoils used in gas turbine engines. In this method, a polyurethane film, which has an adhesive resin on one surface, is attached to the surface of the airfoil.
Merz, in U.S. Pat. No. 4,966,527, teaches the construction of a composite blade which includes a jacket, a part of which is an erosion protective outer coating made of a polyurethane lacquer or a polyurethane elastomer film.
Barbier, et. al., in U.S. Pat. No. 4,990,205, teach construction of a composite blade which includes an outer layer of "molded polyurethane, which enables the outer profile of the blade to be produced with precision and which possesses excellent resistance to erosion by sand."
While the polyurethane coatings provide good erosion resistance on the surfaces of non-metallic composite airfoils, the performance of these components does not completely fulfill the desired lifetime requirements. The polyurethane provides good resistance to erosion, and is a significant improvement over the epoxy matrix material of the composite. However, the polyurethane coating, which is bonded to the surface of the pre-cured epoxy matrix composite substrate, suffers from premature disbonding if the coating is penetrated by unusually large or sharp particles impinging on the surface or the surface of the composite has not been prepared carefully enough.
An alternate form of erosion-resistant coating consists of an epoxy impregnated fiberglass cloth which is applied to the surface of the fiber reinforced epoxy matrix composite article. By using similar epoxy resins for the article and the coating, and curing both in the same operation, an erosion-resistant coating which is integrally bonded to the structure is obtained. This coating overcomes the peeling problems associated with the bonded polyurethane coatings, and is much faster and less costly to apply. Even though its erosion resistance is not as good as the polyurethane materials, the reduced cost and high resistance to peeling can make it a more desirable coating.
What is needed is an erosion resistant coating for fiber reinforced epoxy matrix composite articles which has improved resistance to erosion compared to the prior art epoxy base coatings.
What is also needed is an erosion resistant coating for fiber reinforced epoxy matrix composite articles which has better resistance to disbonding than the prior art bonded polyurethane coatings.
What is still further needed is a method for applying the improved erosion resistant coatings to the surface of fiber reinforced epoxy matrix composite articles.